Rashi: Anything that is close to the ground - such as flies of the bird species, insects such as ants and beetles, worms, and animals with short legs, such as weasels, mice and snails and fish.

2.

Ramban: Based on Targum Onkelus, the Ramban defines 'Sheretz' and 'Remes' as things that move constantly ('she'Ratz), included four-legged 'birds', which move along the ground like insects (as opposed to two-legged birds, which mainly fly.